In a collimating system formed by a pair of quarter pitch gradient index rod (GRIN) lenses, there exist three kinds of assembling misalignments: lateral offset, longitudinal separation and angular tilting. Excess coupling losses so arise. The losses increase quadratically with the misalignment amounts when they are small, depending on the focusing parameter of the GRIN lens. It is possible to reduce the excess losses greatly by choosing an appropriate lens in accordance with the practical assembling conditions.